There are several vehicle operating condition which influence the optimum points for shifting gears in a torque interrupt transmission system. The conditions which have a major impact are, the gross vehicle weight (GVW) and the grade or inclination of the road surface. When these two major components are known it is possible to identify the optimum shift point which will support the current combination of vehicle operating conditions and optimize fuel efficiency. While grade can be measured directly with an inclinometer, GVW is not readily available.
One prior approach to shift point selection employs tables stored in memory that relate shift point to throttle position and vehicle speed. The shift points are not adjusted to reflect vehicle operating conditions such as load or grade. An attempt to incorporate load as a factor in shift point selection is disclosed in U.S. Pat. No. 4,789,936 wherein the nominal shift points stored in a table are corrected for calculated load. The load is calculated based on vehicle velocity and accelerator movement measurements. Unfortunately, the effects of load on shift point decisions is different than the effects of grade, and moreover the determination of load without respect to grade is flawed.